The present invention relates to the vacuum drying of capillary porous bulk materials, primarily grain, and can be used in agricultural, food-processing, woodworking, chemical and other industries.
There are known drying methods for capillary porous bulk materials, including grain, that use preheated drying air interacting with the material subject to drying under fluidization conditions in order to remove hygroscopic moisture (application RF N 93028584, MPK Cl.F26B17/10).
The disadvantage of this method is low process economy due to the high consumption of drying agent, difficulties in organizing control over the material heating temperature and exposure time for separate particles of the material in the reaction zone that influence both material drying time and quality of the material subject to drying.
There are known vacuum drying methods for capillary porous bulk materials, primarily grain, involving the use of a vacuum chamber for the material subject to drying and reduction of pressure in this chamber to 10-30 mm of Hg column using a vacuum pump. The heat is supplied to the grain subject to drying from the ambient air and solar radiation (Patent RF N 2163993, MPK Cl. F26B 5/00, 5/04, 7/00; A01C 1/00; B02B 1/00).
The unit used for this grain vacuum drying method comprises a vacuum chamber made of two tubes coaxially located to each other and mounted vertically in open air connected to the vacuum pump and refrigerator with freezer and condensing units.
The main disadvantage of this method and the unit used for it is that the method is low efficient since heating of the material depends on the environmental conditions and the whole vacuum drying process also become dependent on such conditions, therefore limiting the time of using this method and the unit to the seasons.
The method and device which are the closest by their technical essence and chosen as prototypes are the evaporation vacuum drying method for grain and device used for it (patent RF N 2124294, MPK Cl. A23B 9/00,9/08). The grain is loaded into a vacuum drying chamber that has heating elements and vacuum is created in it. The material subject to drying is additionally heated with the help of the thermal agent that uses condensation energy of the moisture evaporated in the vacuum section of the drying chamber and coming from the other section of the chamber. The grain is being cooled by removing heat from the heat medium coming out from the drying chamber, which, in its turn, is used for preheating grain before it is loaded into the drying chamber.
This method works in the device used for drying grain in vacuum and comprising a vacuum drying chamber divided into steam and grain sections by a louver screen, a heater located in the grain section, inlet and outlet rotary locks, a vacuum pump, a heat-exchanger-cooler united with a heat-exchanger-heater for preheating the grain by pipelines into one closed-loop system and a pipework for heat medium circulation and condensate release. The heater has a panel of tubes with input annular nozzles and output diffusers on each tube, wherein the said panel of tubes is located in a case connected with the steam section of the drying chamber, inputs of the tubes are connected with the heater's outlet and outputs of the tubes—with its inlets via a pump. Water containing surface active agents is used as heat medium.
The disadvantage of this method is that the drying process is performed in a balanced condition, which at low pressure both complicates supply of thermal energy to the material and increases drying time. Besides, the device realizing the said method has a complicated design and requires considerable material costs for non-standard equipment, including a control system.